Generator and method for generating a controlled frequency

ABSTRACT

A frequency generator for generating a controlled signal having a controlled frequency uses a frequency ratio generator with an input; a frequency divider for dividing the controlled frequency by a frequency ratio signal to generate a divided signal having a divided frequency; a converter for generating an excitation signal having the divided frequency, the excitation signal exciting a resonator for generating a resonance signal having a resonance frequency; a frequency phase detector of a phase difference between the divided frequency and the resonance frequency; an inner loop filter for generating the frequency ratio signal and filtering the phase difference signal to prevent instability of two frequency ratio generator loops; an output configured for providing the frequency ratio signal based on a ratio between the controlled frequency and the resonance frequency; and a controlled oscillator circuit for generating the controlled signal based on comparison of the frequency ratio with a target ratio.

FIELD OF THE INVENTION

The invention relates to the field of generators and methods forgenerating a controlled frequency, which is highly stable.

BACKGROUND OF THE INVENTION

Generating a signal having a controlled frequency can be a usefulelectronic building block in different electronic circuits with verydivers functionalities.

A known circuit that generates frequencies is known from WO 2013/066161A1, specifically FIG. 2.

WO 2013/066161 A1 discloses an electronic oscillator circuit, comprisinga first oscillator, for supplying a first oscillation signal, a secondoscillator, for supplying a second oscillation signal, a firstcontroller for delivering the first control signal as a function of aphase difference between a first controller input and a secondcontroller input of the first controller; a second controller fordelivering the second control signal as a function of a phase differencebetween a first controller input of the second controller and a secondcontroller input of the second controller; a resonator; at least asecond resonance frequency, with a first phase shift dependent on thedifference between the frequency of a second exciting signal and thesecond resonance frequency and processing means, for receiving the firstoscillator signal and the second oscillator signal, determining theirmutual proportion, looking up a frequency compensation factor in aprestored table and outputting a compensated oscillation signal.

In FIG. 2 of WO 2013/066161 A1 the divider setting of the feedbackdivider determines the ratio between the resonator frequency and thefrequency output.

A disadvantage of the circuit in FIG. 2 of WO 2013/066161 A1 is that thefrequency is not controllable. Furthermore, a disadvantage is that thecircuit has only one feedback loop. Having only one feedback loop causesall noise sources in the loop to contribute to the phase noise of theoutput frequency. Hence, the loop filter in this circuit must be atrade-off between high bandwidth for tracking changes in the loop andlow bandwidth damping all noise sources in the loop.

US 2014/152354 A1 discloses a method of tuning the frequency of agenerated signal to form an output signal including: forming thegenerated signal at a signal generator; comparing a feedback signal witha reference signal and generating a control signal in dependence on thatcomparison, wherein the feedback signal is generated using the outputsignal; and generating the output signal by performing afrequency-dividing operation in dependence on the generated signal and adividing factor, wherein the dividing factor is determined in dependenceon the control signal. A disadvantage of US 2014/152354 A1 is that theoutput signal may have a phase noise making the method unsuitable fordemanding.

SUMMARY OF THE INVENTION

An object of the invention is to overcome one or more of thedisadvantages mentioned above.

According to a first aspect of the invention, frequency generator forgenerating a controlled signal having a controlled frequency,comprising:

-   -   a frequency ratio generator arranged for generating a frequency        ratio, comprising:    -   an input configured for receiving the controlled signal;    -   a first controlled frequency divider (110) arranged for        generating a first divided signal (115) having a first divided        frequency being substantially the controlled frequency divided        by the first frequency ratio signal;    -   a converter arranged for generating an excitation signal (129)        having the first divided frequency based on the first divided        signal, wherein the excitation signal is provided to the        resonator for excitation of the resonator;    -   connectors for connecting to a resonator arranged for generating        a resonance signal having a first resonance frequency, wherein        the resonator is excited based on the controlled signal; and    -   an output configured for providing a frequency ratio signal        based on a first frequency ratio signal indicative of the        frequency ratio between the controlled frequency and the first        resonance frequency;

wherein the frequency generator further comprises:

-   -   a comparator arranged for generating a comparison signal based        on the comparison of the frequency ratio with a target ratio;        and    -   a controlled oscillator circuit arranged for generating the        controlled signal having the controlled frequency based on the        comparison signal.

The frequency ratio generator outputs the frequency ratio signal. Thefrequency ratio signal is indicative of the frequency ratio between thecontrolled frequency and the first resonance frequency. The resonatorprovides a relatively stable or fixed resonance frequency, which can notbe controlled over a wide range of frequencies. The controlledoscillator on the other hand may provide such a broad range offrequencies or cannot be controlled at all. The frequency ratio issettable at a controlled ratio with the target ratio according to theinvention. Hence, the current invention provides the advantage of acontrolled signal having a controllable frequency over a wide frequencyrange.

The target ratio may be set to any positive rational number. As anexample, the forward error correction in telecommunications necessitatesextra bits to be inserted or extracted from a bitstream. Both bitstreamshave different frequencies and therefore typically their own clocksignals. These clock signals are typically non-integer multiples fromeach other. The current frequency generator advantageously allows forthe generation of non-integer ratios between the controlled frequencyand the resonance frequency.

The comparator handles relatively low frequency signals. These signalsare relatively small band, from DC upwards. The comparator may thereforebe implemented in a DSP or CPU providing the advantage that additionalfeatures may be easily implemented without direct hardware consequences.Hence, the current frequency generator has the advantage to be highlyadaptive to new or additional functionalities.

For a detailed description of the frequency generator used as element inthis invention, reference is made to the text filed as patentapplication NL2022646. This text is hereby incorporated as a whole.Specific reference is made to the embodiments in this text incorporatinga resonator. Also, specific reference is made to definitions and ranges,which also apply to the current invention.

The controlled signal is typically a low phase noise signal, such as inthe range of less than 500 fs, preferably 200 fs, more preferably 100fs, most preferably 80 fs in a frequency range of 8 kHz to 24 MHz,preferably 10 kHz to 22 MHz, more preferably 12 kHz to 20 MHz, mostpreferably 14 kHz to 18 MHz.

In an embodiment of the invention, the frequency generator comprises:

-   -   an outer loop filter arranged for generating a filtered        comparison signal based on the comparison signal;

wherein the controlled frequency is based on the filtered comparisonsignal;

wherein an outer loop is formed by the frequency ratio generator, thecomparator, the comparison signal, the outer loop filter, the filteredcomparison signal, the controlled oscillator and the controlled signal;and

wherein the outer loop filter filters the comparison signal such thatinstability of the outer loop is prevented.

Instability of the loop may be detected from an instable controlledfrequency of the controlled signal. Typically, the frequency ratiogenerator has an inner loop, which is a low-pass filter with a cut-offfrequency. The outer loop filter is typically also a low-pass filter.Furthermore, the cut-off frequency of the outer loop is selected suchlow that the maximum amount of noise is rejected to allow the controlledoscillator to generate the controlled signal with the least amount ofphase noise. On the other hand, the cut-off frequency should be selectedsuch high that changes in the outer loop due to for example temperaturechanges or hysteresis can be tracked such that the controlled frequencystays stable. Furthermore, the cut-off frequency should be selected suchhigh that start-up delay is minimized. Both loop filters typically have,next to a proportional behaviour also an integrating behaviour. Theintegrating behaviour in a static situation or stable mode causes theloop where the loop filter is part of to carry no residual error foradvantageously increasing the accuracy of achieving the target ratio.

In an embodiment of the invention, the comparator comprises a subtractorarranged for providing the comparison signal based on subtracting thetarget ratio from the frequency ratio; and/or

wherein the comparator comprises a divider arranged for providing thecomparison signal based on dividing of the frequency ratio by the targetratio.

The subtractor may comprise and adder with a signal invertor on oneinput. The divider may comprise a multiplier with a reciprocal value fedto one of its inputs. The measured ratio is typically compared to atarget ratio. The target ratio may be predefined, set or continuouslychanged by another feedback loop. The target ratio may be changed, suchas temporarily, continuously or permanently, to align the phase of thecontrolled frequency to the phase of a resonance signal of theresonator.

As the frequency ratio is controlled, the frequency ratio will typicallybe substantially or at least close to the target ratio during stableoperation. Thus, it may be stated that:

t = target  ratio r = frequency  ratio${{t - r} = {{\frac{r}{r}\left( {t - r} \right)} = {{r\left( {\frac{t}{r} - 1} \right)} \approx {\frac{t}{r} - 1}}}},\left. {\lim r}\rightarrow 1 \right.$

The preceding condition is typically satisfied during stable operationand wherein the frequency ratio is normalized in respect to the targetratio. Thus, for stable operation, the subtractor and divider with anoffset of −1 may be seen as identical. The behaviour of the frequencyratio generator and thus also the frequency generator may be differentduring start-up or power-up and during compensating for largerdisturbances in the frequency generator, especially in the differentloops.

In an embodiment of the invention, the frequency ratio generatorcomprises:

-   -   a first controlled frequency divider arranged for generating a        first divided signal having a first divided frequency being        substantially the controlled frequency divided by the first        frequency ratio signal;    -   a converter arranged for generating an excitation signal having        the first divided frequency based on the first divided signal,        wherein the excitation signal is provided to the resonator for        excitation of the resonator;    -   a first frequency phase detector arranged for generating a first        phase difference signal based on a first frequency phase        difference between the first divided frequency and the first        resonance frequency; and    -   a first inner loop filter arranged for generating the first        frequency ratio signal;

wherein a first frequency ratio generator loop is formed by the firstcontrolled frequency divider, the first divided signal, the firstfrequency phase detector, the first phase difference signal, the firstinner loop filter and the first frequency ratio signal;

wherein a second frequency ratio generator loop is formed by the firstcontrolled frequency divider, the first divided signal, the converter,the excitation signal, the resonator, the resonance signal, the firstfrequency phase detector, the first phase difference signal, the firstinner loop filter and the first frequency ratio signal; and

wherein the first inner loop filter filters the first phase differencesignal such that instability of the frequency ratio generator loops areprevented.

The controlled frequency divider, such as the first controlled frequencydivider, may be a digital controlled frequency divider. The controlledfrequency divider may be a controlled multiple frequency divider or acontrolled fractional frequency divider. The frequency phase detector,such as the first frequency phase detector, may be a frequency mixer,analogue multiplier, digital circuit or logic circuit configured asphase detector. Alternatively, the frequency phase detector may be adigital mixer, such as an XOR-port mixer.

The controlled frequency divider divides the frequency of the firstinput signal depending on the control signal, such as the first controlsignal. Typically, the control signal is a stabilized signal having alow jitter to prevent the introduction of significant additional jitterin the controlled frequency divider. The control signal may contain anoffset. The divided signal, such as the first divided signal, typicallycontains the jitter or noise introduced by the controlled frequencydivider as dominant noise source.

The frequency phase detector detecting the phase between a frequency ofthe divided signal and a frequency of the second input signal, such asthe resonance signal, typically also introduces noise. Therefore, thevalue, such as the height, of the phase difference signal is typicallydominated by the phase difference and typically additionally comprisesthe noise introduced by the controlled frequency divider and thefrequency phase detector.

The loop filter, such as the first loop filter, filters the phasedifference signal. The output signal of the loop filter is the controlsignal, which is indicative of the frequency ratio. The loop filter istypically a low-pass filter. The loop filter cut-off frequency isselected based on two limitations. If the cut-off frequency is selectedtoo low, the loop will react too slow to disturbances in the loopcausing instability of the loop. If the cut-off frequency is selectedtoo high, the loop will not reject the noise in the phase differencesignal enough also possibly causing instability of the loop. Instabilityof the loop may be detected from an instable control signal. An instablecontrol signal may have a signal value swinging from one end of therange to another end of its range or be stuck at one of the extremes.The cut-off frequency is typically selected such low that the maximumamount of noise is rejected from the phase difference signal while thecontrol signal is still allowed to adapt quickly enough to anydisturbances introduced in the loop.

The frequency ratio generator provides the advantage of a very stablefrequency ratio measurement. The frequency ratio generator according tothe current invention has a feedback loop having an integrating effect.The integrating effect provides the advantage of a 20 dB/decaderejection of low frequency noise for the phase. This effect in thefrequency domain causes a 40 dB/decade rejection, while known systemsonly have a 20 dB/decade rejection.

Furthermore, the frequency ratio generator provides that the temperaturebehaviour of the filtering components, such as the loop filter, istransposed to nearly DC with the effect of that small deviations onlyslightly changes the effective bandwidth, but does not change the actualcentre frequency. In comparison, known in the art are loop filters,wherein the known loop maintains a certain frequency and thus theseknown loop filters are band-pass filters. If components of the knownloop filter change in value under the influence of temperature or agingfor example, the centre frequency of the known loop filter will change.Therefore, the circuit according to the current invention provides theadvantage of an increased temperature stability.

This frequency ratio generator is having a resonator providing theresonance signal and providing an excitation signal to the resonatorbased on the divided signal. As further detailed below the circuitprovides the advantage of not having to comply to the Barkhausencriterium. This not complying to the Barkhausen criterium has the effectof reducing the phase noise.

Prior art loops complying to the Barkhausen criterium may be compared toan AM-signal transmission. The current invention, not complying to theBarkhausen criterium may be compared to an FM-signal transmission.FM-signals are known to be less prone to disturbances. One of suchdisturbances in the current invention may be a small and adjacentresonance frequency. These small and adjacent resonance frequencies mayeven cross-over, for example under the influence of a temperaturechange. Especially in these cases not having to comply to the Barkhausencriterium provides the advantage of a considerable rejection of thenoise generated by the small and adjacent resonance frequency.

Generating particular frequencies with low phase noise requires asubstantial amount of power. This frequency generator provides theadvantage of two independent frequencies, the controlled frequency andthe resonance frequency, loosely coupled via the frequency ratio andboth with low phase noise without doubling the power, but with much lesspower consumed by the circuit.

In an embodiment of the invention, the phase difference signal has amagnitude, such as an amplitude or a value, based on the frequency phasedifference and/or the control signal has a magnitude, such as anamplitude or a value, indicative of the frequency ratio. If the phase orcontrol signal is an analogue signal, the signal information istypically advantageously contained in the amplitude of the signal. Ifthe phase or control signal is a digital signal, the signal informationis typically advantageously contained in a digital value of the signal.The digital value may be a binary code, BCD code, Gray code orcombination of these codes or any other code having a defined valuesystem.

In an embodiment of the invention, the loop filter is a lowpass filter,preferably having a cut-off frequency advantageously below the frequencynoise introduced by the controlled frequency divider and preferableadvantageously above a rate of change of the first and secondfrequencies. Furthermore, the cut-off frequency should be advantageouslyselected such that the control signal is inside a specified range, whilethe circuit remains stable under changing conditions causingdisturbances in the circuit. The specified range is typically userspecified. The specified range may also be determined based on the noiseintroduced by the other features in the circuit, such as the controlledfrequency divider and the frequency phase detector. The specified rangemay also be influenced by the variations of the first input signal andthe second input signal.

In an embodiment of the invention, the controlled frequency divider is acontrolled fractional frequency divider. A phase difference between thedivided signal and the second input signal may be caused by a staticphase difference and/or a frequency difference and lapsing time. In thisembodiment, the use of a controlled fractional frequency divideradvantageously allows for improved frequency matching between thefrequency of the divided signal and the second frequency because thefirst frequency is divided with a higher granularity. Furthermore, asthe granularity is higher, the first frequency may advantageously beselected lower. The use of high frequencies has the disadvantage ofcross-talk, increased energy loss, etc.

The resonator may be a crystal resonator and preferably the resonancefrequency is an overtone resonance frequency of the crystal resonator.The resonator is typically a resonator allowing resonances of differentfrequencies at the same time. A crystal is a readably available solutionfor a resonator. Harmonic frequencies change in frequency under theinfluence of temperature in a similar manner as the fundamentalfrequency. In contrast, overtone frequencies may change frequency in adifferent manner under the influence of temperature. Furthermore,different overtones of a crystal may change in frequency in a differentmanner under the influence of temperature. Thus, different overtones mayhave different temperature gradients. As an overtone of the crystal maybe selected, the circuit can advantageously be designed to have apredefined temperature behaviour based on the predefined behaviour ofthe resonator.

As described, if at least one, but preferably two of the dividedfrequencies are selected at an overtone frequency, the temperaturebehaviour may be different. The temperature may vary in a temperaturerange of −40° C. to +125° C. Within this range the temperature gradientfor a certain overtone may vary.

In an embodiment of the invention, the frequency ratio generatorcomprises:

-   -   a second controlled frequency divider arranged for generating a        second divided signal having a second divided frequency being        substantially the controlled frequency divided by a second        frequency ratio signal;    -   a second frequency phase detector arranged for generating a        second phase difference signal based on a second frequency phase        difference between the second divided frequency and the second        resonance frequency;    -   a second inner loop filter arranged for generating the second        frequency ratio signal indicative of the second frequency ratio        based on the second phase difference signal; —an adder arranged        for generating an added signal supplied to the converter,        wherein the added signal has the first divided frequency and the        second divided frequency;    -   a temperature compensator for compensating changes in resonance        frequency of the resonator, wherein the temperature compensator        comprises:    -   an input configured for receiving the first frequency ratio and        the second frequency ratio;    -   a first divider arranged for generating a first division value        based on dividing of the first frequency ratio and the second        frequency ratio;    -   a calculator arranged for calculating a compensation factor        based on the value of one or more of the group of the first        phase difference signal, the second phase difference signal, the        first frequency ratio and the first division value, wherein the        calculator comprises preferably a look-up table; and    -   a redresser arranged for generating the frequency ratio based on        redressing a signal based on one or more of the group of the        first phase difference signal, the second phase difference        signal and the first frequency ratio with the compensation        factor;

wherein the excitation signal is based on the added signal;

wherein a third frequency ratio generator loop is formed by the secondcontrolled frequency divider, the second divided signal, the secondfrequency phase detector, the second phase difference signal, the secondinner loop filter and the second frequency ratio signal;

wherein a fourth frequency ratio generator loop is formed by the secondcontrolled frequency divider, the second divided signal, the converter,the excitation signal, the resonator, the resonance signal, the secondfrequency phase detector, the second phase difference signal, the secondinner loop filter and the second frequency ratio signal;

wherein the second inner loop filter filters the second phase differencesignal such that instability of the frequency ratio generator loops isprevented.

The frequency ratio generator loops are parallel and are evaluatedsimultaneously, the temperature effects are measured at the same time.Known circuits may have the tendency to measure temperature effectsspaced in time. This time spacing causes temperature inaccuracies inknown circuits if the temperature changes over time. The ratio generatoraccording to the current invention has therefore the advantage ofimproved accuracy of the temperature measurement.

In an embodiment of the invention, the frequency ratio generatorcomprises:

-   -   a selector arranged for selecting one or more of the group of        the first phase difference signal, the second phase difference        signal and the first frequency ratio based on the value of one        or more of the group of the first frequency ratio and the first        division value; and

wherein the redresser is arranged for generating the frequency ratiobased on redressing the selected frequency ratio with the compensationfactor.

This embodiment of the ratio frequency generator advantageously uses theproperty of the resonator that the resonator may resonate at multiplefrequencies at the same time. Resonators are typically bulky compared tothe other elements of the circuit. Thus, using the resonator to resonateat multiple frequencies allows integration and miniaturization of thecircuit.

Furthermore, the selector advantageously may select the frequency ratiomost stable for a certain temperature such that the frequency ratio doesnot change much due to temperature changes. Furthermore, the redressermay base the compensation factor predominantly on the parameter havingthe highest change of value for a certain temperature range. Theparameters may also be used to determine the temperature.

In an embodiment of the invention, the frequency ratio generatorcomprises:

-   -   a third controlled frequency divider arranged for generating a        third divided signal having a third divided frequency being        substantially the controlled frequency divided by a third        frequency ratio signal;    -   a third frequency phase detector arranged for generating a third        phase difference signal based on a third frequency phase        difference between the third divided frequency and the third        resonance frequency;    -   a third inner loop filter arranged for generating the third        frequency ratio signal indicative of the third frequency ratio        based on the third phase difference signal;

wherein the added signal also has the third divided frequency;

wherein the input of the temperature compensator is further configuredfor receiving the third frequency ratio; and

wherein the temperature compensator further comprises:

-   -   a second divider arranged for generating a second division value        based on dividing of the first frequency ratio and the third        frequency ratio;

wherein, when depending on embodiment 6, the selector is furtherarranged for selecting also from the group of the third phase differencesignal, the second frequency ratio and the third frequency ratio andalso based on the value of one or more of the extended group of thesecond frequency ratio, the third frequency ratio and the seconddivision value;

wherein the calculator is further arranged for calculating thecompensation factor based on the value of one or more of the groupextended with the third phase difference signal, the second frequencyratio, the third frequency ratio and the second division value;

wherein the redresser is arranged for generating the frequency ratiobased on redressing the selected frequency ratio with the compensationfactor;

wherein a fifth frequency ratio generator loop is formed by the thirdcontrolled frequency divider, the third divided signal, the thirdfrequency phase detector, the third phase difference signal, the thirdinner loop filter and the third frequency ratio signal;

wherein a sixth frequency ratio generator loop is formed by the thirdcontrolled frequency divider, the third divided signal, the converter,the excitation signal, the resonator, the resonance signal, the thirdfrequency phase detector, the third phase difference signal, the thirdinner loop filter and the third frequency ratio signal; and

wherein the third inner loop filter filters the third phase differencesignal such that instability of the frequency ratio generator loops isprevented.

This embodiment advantageously allows to measure hysteresis behaviour ofthe resonator during operation. Known circuits tend to compensate forhysteresis in the resonator by designing in countermeasures from atheoretical point. Hence, this embodiment provides the advantage ofimproved accuracy due to hysteresis measurement.

In an embodiment of the invention, the selector of the frequency ratiogenerator is arranged for:

-   -   selecting one of the group of the first frequency ratio, the        second frequency ratio and the third frequency ratio, wherein        the selected ratio is based on the value of one or more of the        group of the first frequency ratio, the second frequency ratio,        the third frequency ratio, the first division value and the        second division value; or    -   selecting a weighted combination of two or more of the first        frequency ratio, the second frequency ratio and the third        frequency ratio, wherein the weighted combination is based on        the value of one or more of the group of the first frequency        ratio, the second frequency ratio, the third frequency ratio,        the first division value and the second division value.

The previously mentioned advantages for the selector apply also to thismore elaborate embodiment of the selector. Furthermore, this version ofthe selector allows for a more balanced combination with the weightedcombination. For example, the weighted combination allows advantageouslyfor mixing and matching of different performance aspects, such as Allanvariance or phase noise.

In an embodiment of the invention, the first division value is solelybased on the first frequency ratio signal and the second frequency ratiosignal;

the second division value is solely based on the first frequency ratiosignal and the third frequency ratio signal;

the selector is arranged for selecting one or more of the limited groupof the first phase difference signal, the second phase difference signaland the third phase difference signal; and/or

the calculator is arranged for calculating a compensation factor basedon the value of one or more of the limited group of the first divisionvalue and the second division value. This embodiment advantageouslylimits the amount of choices in the different groups to the signalsand/or ratios best suitable for providing the compensation factor, suchas for changes in parameters of the different components due totemperature changes.

In an embodiment of the invention, the frequency ratio generatorcomprises an analogue to digital converter arranged for generating adigital resonance signal based on the resonance signal, wherein thedigital resonance signal is supplied to at least the first frequencyphase detector. Depending on the operation, manipulating input signalsin the digital or analogue domain may advantageously be done in one ofthe domains. Typically, the resonator is an analogue component readilyavailable. Further typically, a frequency phase detector is easier toimplement in the digital domain. The ADC advantageously provides acoupling of the analogue and digital domain to benefit from theavailability on one hand and ease of implementation on the other hand.

In an embodiment of the invention, the converter of the frequency ratiogenerator comprises a digital to analogue converter arranged forgenerating the excitation signal based on the divided signal. A digitalcontrolled frequency divider is simpler in implementation and can beeasily controlled. The resonator is an analogue component. Adding a DACbetween the digital controlled frequency divider and the resonatorprovides the advantage of coupling two advantageous partial solutions inthe digital and analogue domain respectively.

In an embodiment of the invention, the comparator comprises:

-   -   a scaler arranged for generating a scaled signal, which is the        frequency ratio signal scaled by a scaling factor; and/or    -   a shifter arranged for generating a shifted signal, which is the        scaled signal shifted by a shift value;

wherein the comparison signal is based on the shifted signal.

The controlled oscillator typically has an input for controlling thecontrolled frequency of the controlled signal. The input signal, fedinto the input of the controlled oscillator, is based on the comparisonsignal. Depending on the embodiment, other signals may be combined, suchas added, subtracted, divided and multiplied, with the comparisonsignal. The comparison signal may also be manipulated, such as filtered,before becoming the input signal. Typically, if the input to thecontrolled oscillator is zero, the controlled frequency will have acertain ground frequency or centre frequency. And any deviation fromzero in positive or negative direction would result in a positive ornegative frequency change, respectively. Applying a scaler and shifterprovides the option to manipulate the signal such that the signal issuitable as an input signal for most common controlled oscillators.Furthermore, the scaler and shifter provide the advantage of introducingan additional offset. The additional offset may for example be used in atelecommunication system requiring some extra space for insertingsignalling into a bitstream.

The controlled oscillator may be implemented in many differentembodiments. The controlled oscillator may be a voltage-controlledoscillator, optionally with the output of a DAC providing the signal forthe control input of the voltage-controlled oscillator. The controlledoscillator may also be an all-digital PLL.

In an embodiment of the invention, the frequency generator comprises:

-   -   a phase acquisition circuit arranged for generating a phase        delta signal based on the phase difference between the        controlled frequency and a reference signal having a reference        frequency; and    -   a PLL arranged for generating an offset signal based on the        phase delta signal;

wherein the controlled frequency is also based on the offset signal.

Adding the PLL, preferably digital PLL, allows for phase and frequencytracking of an external signal having a particular frequency. Theexternal signal may have a considerable phase jitter, such as a carrierwave for a telecommunications protocol. The frequency generator mayadvantageously generate a very stable controlled signal having a verystable controlled frequency. The controlled frequency is typicallycontrolled such that the controlled frequency may be used as referencewhile receiving the communication carried by the carrier wave.

An advantage of this embodiment, when implemented for the larger part inthe digital domain, is that the frequency generator allows for thedifferent noise sources, such as the external signal, the resonator andthe controlled oscillator, to be identified and at least partlycompensated for through the architecture, such as the loops,specifically the loop filters, of the frequency generator. For example,the resonator is typically influenced by temperature, causing a changein resonance in the range of a 10's of kHz. This while the controlledoscillator typically creates phase noise in the multi 100 kHz or evenMHz range which may be due to Fermi sea, Brownian motion, etc.

The resonator and the controlled oscillator are typically elementsimplemented in the analogue domain. The larger part of the frequencygenerator, advantageously implemented in the digital domain, may requireimplementation in dedicated hardware, but may also allow a part of theimplementation done in software.

Loops comprising a resonator or oscillator commonly adhere to theBarkhausen criterium. The Barkhausen criterium contains the followingconstrains:

the absolute magnitude of the gain of the loop equals 1; and

the phase shift of the loop is 2πx; x∈

₀ ⁺

The Barkhausen criterium typically implies additional design constrainsfor the resonator loop. A loop complying to the Barkhausen criterium istypically hard to design and introduces additional phase noise.Typically, a loop comprising a regular amplifier for making the loopcomply to the Barkhausen criterium introduces considerable phase noiseand consumes considerable power. The controlled oscillator is the onlyelement in the frequency generator, which may comply to the Barkhausencriterium. Therefore, this embodiment provides the advantage of lowpower and low amount of phase noise introduction.

In a further embodiment of the invention:

-   -   when comprising a shift value, the shift value is the offset        signal; or    -   when comprising a filtered comparison signal, the filtered        comparison signal is indirectly based on the offset signal.

In the first option the offset signal is fed through the outer loopfilter, while in the second option the offset signal is not fed throughthe outer loop filter. In the context of this text, the word directlyand indirectly based on for a signal mean if the input signalcontributes to the resulting signal or via a loop, respectively. Thefirst option advantageously limits the number of filters in the system.The second option advantageously allows the offset signal to be filteredor unfiltered added to the signal controlling the controlled oscillator.The filter applied to the offset signal may advantageously be designedto the particular needs for filtering the offset signal. This isparticularly advantageous as the offset signal typically has thecontrolled oscillator as primary noise source, having a noise in themulti 100 kHz or even MHz band, while the filtered comparison signal hastypically the resonator as primary noise source, having a noise in the10-s of Hz band.

According to another aspect of the invention, a method for generating acontrolled signal having a controlled frequency, comprising the stepsof:

-   -   receiving a resonance signal having a first resonance frequency        from a resonator;    -   providing a first ratio signal indicative of the first frequency        ratio signal between the controlled frequency and the first        resonance frequency;    -   generating a first phase difference signal (155) based on a        first frequency phase difference between a first divided        frequency and a first resonance frequency;    -   filtering the first phase difference signal for generating the        first frequency ratio signal;    -   basing a frequency ratio on the first frequency ratio signal;    -   providing a target ratio;    -   generating a comparison signal based on the comparison of the        frequency ratio with the target ratio;    -   generating the controlled signal having the controlled frequency        based on the comparison signal;    -   generating a first divided signal (115) having a first divided        frequency being substantially the controlled frequency divided        by the first frequency ratio signal;    -   generating an excitation signal (129) having the first divided        frequency based on the first divided signal, wherein the        excitation signal is provided to the resonator for excitation of        the resonator; and    -   outputting the controlled signal;

wherein a first frequency ratio generator loop is formed by the firstdivided signal, the first phase difference signal and the firstfrequency ratio signal;

wherein a second frequency ratio generator loop is formed by the firstdivided signal, the excitation signal, the resonator, the resonancesignal, the first phase difference signal and the first frequency ratiosignal;

wherein the step of filtering filters the first phase difference signalsuch that instability of the frequency ratio generator loops areprevented.

According to another aspect of the invention, a frequency generatorsystem comprising:

-   -   a frequency generator chip comprising a frequency generator        according to any of the preceding embodiments;    -   a resonator for connecting to the frequency generator chip for        generating the resonance signal; and    -   an oscillator for connecting to the controlled oscillator        circuit for generating an oscillating signal.

According to another aspect of the invention, a computer program productcomprising a computer readable medium having computer readable codeembodied therein, the computer readable code being configured such that,on execution by a suitable computer or processor, the computer orprocessor is caused to perform the steps of the method in an embodimentaccording to the invention:

-   -   providing a first ratio signal;    -   generating a comparison signal;    -   generating the controlled signal;    -   outputting an excitation signal; and    -   outputting the controlled signal.

According to another aspect of the invention, a computer program productcomprising a computer readable medium having computer readable codeembodied therein, the computer readable code being configured such that,on execution by a suitable computer or processor, the computer orprocessor is caused to perform the method specified in an embodimentaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be apparent from and elucidated further withreference to the embodiments described by way of example in thefollowing description and with reference to the accompanying drawings,in which:

FIG. 1 schematically shows a first embodiment of the frequencygenerator;

FIG. 2 schematically shows a second embodiment of the frequencygenerator;

FIG. 3 schematically shows a first embodiment of the frequency ratiogenerator;

FIG. 4 schematically shows a second embodiment of a frequency ratiogenerator;

FIG. 5 schematically shows a third embodiment of a frequency ratiogenerator;

FIG. 6 schematically shows a sub-system for compensating temperatureeffects;

FIG. 7 schematically shows a sub-system for compensating temperature andhysteresis effects; and

FIG. 8 schematically shows an embodiment of a computer program product.

The figures are purely diagrammatic and not drawn to scale. In thefigures, elements which correspond to elements already described mayhave the same reference numerals.

LIST OF REFERENCE NUMERALS

 100 first embodiment frequency ratio generator  101 second embodimentfrequency ratio generator  102 third embodiment frequency ratiogenerator  104 first input signal  105 third input signal  106 fourthinput signal  107 (first) control signal  108 second control signal  109third control signal  110 (first) controlled frequency divider  111second controlled frequency divider  112 third controlled frequencydivider  115 (first) divided signal  116 second divided signal  117third divided signal  120 adder  121 added signal  125 digital toanalogue converter (DAC)  129 excitation signal  130 resonator  135second input signal  140 analogue to digital converter (ADC)  145digital second signal  150 (first) frequency phase detector  151 secondfrequency phase detector  152 third frequency phase detector  155(first) phase difference signal  156 second phase difference signal  157third phase difference signal  160 (first) loop filter  161 second loopfilter  162 third loop filter  200 first embodiment sub-system  201second embodiment sub-system  210 first circuit comprising resonator 211 second circuit comprising resonator  220 (first) divider  221second divider  225 first indication  226 second indication  230subtractor  235 subtracted signal  300 first embodiment frequencygenerator  301 second embodiment frequency generator  310 comparator 311 target ratio  312 frequency ratio signal  315 comparison signal 320 outer loop filter  321 filtered comparison signal  330 controlledoscillator circuit  331 controlled signal  340 phase acquisition circuit 341 reference signal  345 phase delta signal  350 digital PLL  355offset signal  356 adder  357 oscillator control signal 1000 computerprogram product 1010 computer readable medium 1020 computer readablecode

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following figures may detail different embodiments. Embodiments canbe combined to reach an enhanced or improved technical effect. Thesecombined embodiments may be mentioned explicitly throughout the text,may be hint upon in the text or may be implicit.

FIG. 1 schematically shows a first embodiment of the frequency generator300. The frequency generator comprises a frequency ratio generator 100,101, 102, a comparator 310 and a controlled oscillator 330.

The frequency ratio generator has as input the controlled frequency 331.The controlled frequency is the generated frequency and thus effectivelythe output of the frequency generator. The frequency ratio generatorprovides a frequency ratio signal 312.

The frequency ratio generator comprises a resonator. The frequency ratiogenerator determines a ratio between the controlled frequency and aresonance frequency of the resonator. Further, the frequency ratiogenerator may determine a ratio between two resonance frequencies of theresonator, wherein at least one of the resonance frequencies is anovertone frequency. Typically, the resonator changes over temperatureand shows the effects of hysteresis. The frequency ratio signal may betemperature stabilized or compensated. The frequency ratio signal may behysteresis stabilized or compensated. The frequency ratio signal may bestabilized or compensated for any other effects of the resonator or anyother component in the frequency ratio generator or in the frequencygenerator.

The comparator has as input the frequency ratio signal and a targetratio 311. The target ratio has typically a predefined value or isselected from a set of predefined values. The predefined values aretypically determined by the manufacturer, preferably duringmanufacturing of the frequency ratio generator or resonator by inlinecharacterisation. The comparator compares the frequency ratio signal tothe target ratio and provides as output a comparison signal 315, whichis the result of this comparison. Typically, the comparison of thefrequency ratio signal and the target ratio is performed by subtraction.

The controlled oscillator has as input a signal based on the comparisonsignal. The input signal may be a filtered comparison signal with offset357, a filtered comparison signal 321, the comparison signal or anyother signal based on the comparison signal. The output of thecontrolled oscillator is the controlled frequency 331.

The controlled oscillator is typically an oscillator with a centrefrequency. The controlled oscillator is typically highly stable forhysteresis. Typically, the controlled frequency equals centre frequencyif the oscillator control signal is 0. Any change of the oscillatorcontrol signal, negative or positive, changes the controlled frequency.The change of the controlled frequency is typically linear over apredefined range. The scaling and shifting to obtain a signal suitableas input for the controlled oscillator is advantageously done in thecomparator. The controlled oscillator may be implemented with the use ofvaricaps, LC-network or may be implemented completely in the digitaldomain.

The frequency generator optionally comprises an outer loop filter 320.The outer loop filter takes the comparison signal as input. The outerloop filter outputs a filtered comparison signal 321. The outer loopfilter filters the comparison signal such that instability of the outerloop is prevented. The outer loop is formed by at least the frequencyratio generator, the comparator, the outer loop filter and thecontrolled oscillator.

The frequency generator optionally comprises an adder 356. The addertakes as a first input the filtered comparison signal, the comparisonsignal or a signal based on the comparison signal. The adder takes as asecond input an offset signal 355. The adder adds the two inputs signalsto provide an added signal. The oscillator control signal may be equalto the added signal or may be based on the added signal. This offsetsignal may be used to shift the added signal to a suitable range asinput for the controlled oscillator. The offset signal may also be usedto introduce an offset on the oscillator control signal to control thecontrolled frequency. The offset signal may also be used to introduce anoffset, typically a temporal offset, on the oscillator control signal tocontrol the phase of the controlled frequency.

Suppose the frequency ratio generator is stable and a zero offset signalis provided. Furthermore, suppose the controlled frequency is slightlytoo high. The frequency ratio generator will output a frequency ratiosignal signalling the ratio between a resonance frequency of theresonator and the controlled frequency. This frequency ratio will beslightly too high. The comparator will compare the target ratio with thefrequency ratio and will conclude that the frequency ratio is slightlytoo high. The result of this comparison will be shown in the comparisonsignal. Typically, the comparison signal will be slightly too lowcompared to its desired settling point. The optional outer loop filtermay filter the comparison signal. If the outer loop filter comprises anintegrating function, the controlled frequency error may be reduced tozero. A signal based on the comparison signal is provided to thecontrolled oscillator. The controlled oscillator will react to theslightly too low signal based on the slightly too low comparison signalby lowering the controlled frequency, thereby stabilizing the controlledfrequency to a desired frequency. The controlled frequency is at leastpartly determined by the target ratio setting. The optional offsetsignal provides an additional means to control the controlled frequency.The offset signal may be injected in the loop before or after theoptional outer loop filter.

The embodiment of the invention in FIG. 1 may use the embodiment of thefrequency ratio generator shown in FIG. 3.

The embodiment of the invention in FIG. 1 may use the embodiment of thefrequency ratio generator shown in FIG. 4 combined with the sub-systemshown in FIG. 6 to provide a temperature compensated frequencygenerator. The frequency generator is especially insensitive totemperature variations when the frequency ratio generator is temperaturecompensated and the other parts of the frequency generator are as muchas possible implemented in the digital domain.

The embodiment of the invention in FIG. 1 may use the embodiment of thefrequency ratio generator shown in FIG. 5 combined with the sub-systemshown in FIG. 7 to provide a temperature and hysteresis compensatedfrequency generator. The frequency generator is especially insensitiveto temperature variations and hysteresis when the frequency ratiogenerator is temperature and hysteresis compensated and the other partsof the frequency generator are as much as possible implemented in thedigital domain.

FIG. 2 schematically shows a second embodiment of the frequencygenerator 301. The frequency generator comprises the elements, signalsand features of the first embodiment. Further, the frequency generatorcomprises a phase acquisition circuit 340 and a PLL 350.

The phase acquisition circuit has the controlled signal having acontrolled frequency and a reference signal 341 having a referencefrequency as input. The phase acquisition circuit determines the phasedifference between the controlled frequency and the reference frequencyand outputs this difference as a phase delta signal 345.

The phase acquisition block may be implemented as a counter, wherein oneinput is used as clock signal to count zero-crossings of the othersignal. The number of crossings relates back to the phase between thesignals.

The PLL has the controlled signal and the phase delta signal as input.The PLL generates a phase lock signal based upon the controlled signaland the phase delta signal. The phase lock signal may be seen as theoffset signal 355. The PLL is preferably a digital PLL or all digitalPLL. The controlled frequency signal is typically used as clock input tothe digital PLL.

The addition of the phase acquisition circuit and the digital PLL locksphase of the controlled frequency to the phase of the referencefrequency. This embodiment is typically used for tracking a remotelygenerated reference frequency and thereafter locally stabilizing thisreference frequency such that local reference in the form of acontrolled frequency exhibits extreme low frequency jitter. Thisfrequency generator therefore compensates or filters out anydisturbances injected between the source of the reference signal and thefrequency generator. Exemplary applications may be found intelecommunication, such as in smartphones or satellite phones, printedcircuit boards, such as larger printed circuit boards, systems syncingto atom clocks, navigation systems, etc.

The scenario as described above for explaining the stabilizingfunctionality of the outer loop may be applied in a similar way to theembodiment of the frequency generator shown in FIG. 2.

Further, suppose the frequency ratio generator is stable and a zerooffset signal is provided. Furthermore, suppose the controlled frequencyis slightly lagging relative to the reference signal. The phaseacquisition block will detect the phase difference between thecontrolled frequency and the reference frequency. The PLL will filtertypically with a high Q-factor the phase delta signal representing thephase difference. The resulting signal from the PLL will be injected asan offset signal in the outer loop for increasing the controlledfrequency. As soon as the phase acquisition circuit detects no phasedifference between the phase delta signal will indicate the absence ofthe phase difference. The resulting signal from the PLL and injected asthe offset signal in the outer loop will decrease the controlledfrequency to stay in phase with the reference signal. Thus, the phase ofthe controlled frequency will be locked onto the phase of the referencesignal. The filtering of the PLL, typically with a high Q value, willreject phase jitter in the reference signal. Hence, this embodimentprovides the advantage of providing a controlled frequency with low,such as extremely low, phase jitter. This frequency generator thereforeallows to filter out any disturbances, such as jitter, of the referencefrequency for providing a local control signal with a controlledfrequency with very stable frequency and low phase jitter.

The embodiment of the invention in FIG. 2 may use the embodiment of thefrequency ratio generator shown in FIG. 3.

The embodiment of the invention in FIG. 2 may use the embodiment of thefrequency ratio generator shown in FIG. 4 combined with the sub-systemshown in FIG. 6 to provide a temperature compensated frequencygenerator. The frequency generator is especially insensitive totemperature variations when the frequency ratio generator is temperaturecompensated and the other parts of the frequency generator are as muchas possible implemented in the digital domain.

The embodiment of the invention in FIG. 2 may use the embodiment of thefrequency ratio generator shown in FIG. 5 combined with the sub-systemshown in FIG. 7 to provide a temperature and hysteresis compensatedfrequency generator. The frequency generator is especially insensitiveto temperature variations and hysteresis when the frequency ratiogenerator is temperature and hysteresis compensated and the other partsof the frequency generator are as much as possible implemented in thedigital domain.

FIG. 3 schematically shows a first embodiment of the frequency ratiogenerator 100. The frequency ratio generator comprises a controlledfrequency divider 110, a frequency phase detector 150 and an inner loopfilter 160. The controlled frequency divider may also be labelled firstcontrolled frequency divider. The frequency phase detector can also belabelled phase detector, first phase detector or first frequency phasedetector. The inner loop filter may also be labelled first inner loopfilter, first loop filter, or loop filter.

The controlled frequency divider takes as inputs a first input signal104 and a control signal 107 and provides as output a divided signal115. Typically, the first input signal is the controlled signal 331. Thecontrol signal may also be labelled first control signal. The dividedsignal may also be labelled first divided signal. The first input signalis a periodic signal having a first frequency, typically the controlledfrequency. The control signal is typically a signal with a large amountof energy in the lower frequencies, such as substantially close to 0 Hz.

The controlled frequency divider generates the divided signal. Thedivided signal is a periodic signal having a divided frequency. Thedivided frequency relates to the first frequency based on the magnitudeof the control signal. The magnitude of a signal may be an amplitude ofthe signal, a value of the signal or any other property of the signalexpressing a measure. In case the control signal is an analogue signal,the magnitude is typically an amplitude of the signal. In case thecontrol signal is a digital signal, the magnitude is typically a valueof the signal. Typically, the relation between inputs and outputs of thecontrolled frequency divider may be linearized to

${{f_{out} = \frac{f_{in}}{x - a}};{{x - a} \in {\mathbb{Z}}^{+}}},{a \in {\mathbb{Z}}}$wherein x is the magnitude of the control signal and a is an offset. Inpractical implementations the set of numbers for x and x-a are selectedmuch more limited.

The phase detector takes as inputs the divided signal and a second inputsignal 135 and provides as output a first phase difference signal 155.The second input signal is a periodic signal having a second frequency.The first phase difference signal may also be labelled phase differencesignal.

The magnitude of the phase difference signal relates to the phasedifference between divided frequency and the second frequency.Typically, depending on the implementation of the phase detector, themagnitude of the phase difference signal may have a minimum at 0degrees, −90 degrees or 90 degrees phase difference.

The loop filter takes as input the phase difference signal and providesas output the control signal. The loop filter is typically a low-passfilter. The loop filter stabilizes the loop or feedback loop formed bythe controlled frequency divider, the divided signal, the frequencyphase detector, the phase difference signal, the loop filter and thecontrol signal. The first control signal may be outputted as a frequencyratio signal 312.

Suppose the first frequency is not changing. Furthermore, suppose thedivided frequency is slightly higher compared to the second frequencyand that the divided signal and the second input signal are in phase.The phase detector will detect an increasing phase difference betweenthe two signals as the second input signal will start lagging behindcompared to the divided signal. The increasing phase difference willcause the magnitude of the phase difference signal to increase. Withsome delay, damping and/or reduction due to being implemented aslow-pass filter, the loop filter will increase the magnitude of thecontrol signal. The increase in control signal will cause the firstfrequency to be divided by a larger magnitude, thus a higher number, toprovide a lower divided frequency. Thus, any difference in frequencybetween the second frequency and the divided frequency is reduced and/orminimized with the negative feedback loop. Furthermore, as the dividedfrequency tracks the second frequency, the magnitude of the controlsignal will be indicative of the ratio between the first frequency andthe second frequency.

In another scenario, suppose the first frequency is increasing.Furthermore, suppose the second frequency is stable. As the firstfrequency is increasing and the magnitude of the control signal isstable, the divided frequency will increase. The phase detector willdetect an increasing phase difference between the two signals as thesecond input signal will start lagging behind compared to the dividedsignal. The increasing phase difference will cause the magnitude of thephase difference signal to increase. With some delay, damping and/orreduction due to being implemented as low-pass filter, the loop filterwill increase the magnitude of the control signal. The increase incontrol signal will cause the first frequency to be divided by a largermagnitude, thus a higher number, to provide a lower divided frequency,which will be substantially the divided frequency before the increase ofthe first frequency. Thus, any changing first frequency will cause thedivided frequency to remain substantially the same to the secondfrequency due to the negative feedback loop. Furthermore, as the dividedfrequency tracks the second frequency, the magnitude of the controlsignal will be indicative of the ratio between the first frequency andthe second frequency, which will be increasing in this situation.

As typically the first and second frequency are both changing, acombination of the scenarios above is likely.

In order to get initial lock, the divided frequency needs to berelatively close to the second frequency, such as the resonancefrequency of a resonator generating the second frequency, otherwise thelock procedure to get initial lock may be quite complex and lengthy intime. If the first or second frequency or combination of the first andsecond frequency move faster than the resonator loop can track, lock maybe lost. Preferably, the first frequency should not change too fast toallow a faster change of the second frequency. Slow frequency movementsof the first and second frequencies over a relatively large range allowlock to be maintained. Fast frequency movements of the first and secondfrequencies over a relatively small range also allow lock to bemaintained.

For some circuits it may be known how much the frequency moves.Combining that knowledge with the circuit allows the selection of aresonator and design of the other elements in the loop for keeping lockduring operation. As a rule of thumb, if the dynamic frequency trackingof the loop is slower than the combination of the change of the firstand second frequencies, lock will be lost.

FIG. 4 schematically shows a second embodiment of a frequency ratiogenerator 101. The circuit comprises all features as described for FIG.3. The circuit may further comprise a second controlled frequencydivider 111, an adder 120, a DAC 125, a resonator 130, an ADC 140, asecond phase detector 151 and a second loop filter 161.

The first controlled frequency 110 divider takes as inputs the firstinput signal 105 and the first control signal 108 and provides as outputthe first divided signal 115. The second controlled frequency dividertakes as inputs a third input signal 105 and a second control signal 108and provides as output a second divided signal 116. The adder takes asinputs the first and second divided signals and provides as output anadded signal 121. The added signal is the addition of the first andsecond divided signals.

The optional DAC takes as input the added signal and provides as outputan excitation signal 129 suitable for letting the resonator resonate.This provides the advantage of having much of the features of thecircuit in the digital domain, while only needing a single DAC forproviding an excitation signal, which is typically an analogue signal,to the resonator, which is typically a crystal or crystal oscillator.Furthermore, frequency dividers are typically implemented in the digitaldomain, providing the advantages of ease of implementation andintroduction of limited phase noise. The introduction of limited phasenoise is substantially due to that a controlled digital fractionalfrequency divider has a higher granularity compared to an integerfrequency divider.

In an alternative embodiment, two DAC-s are present at respective inputsof the adder, such that the adder is an analogue adder. In anotheralternative embodiment, the circuit has no DAC in the loop. In evenanother embodiment, a DAC is between the first loop filter output andthe first controlled frequency divider, preferably a second DAC isbetween the second loop filter output and the second controlledfrequency divider.

The optional ADC takes as input the second input signal and provides asoutput a digital second signal 145. This provides the advantage ofhaving much of the features of the circuit in the digital domain, whileonly needing a single ADC for receiving the second input signal, whichis typically an analogue signal, from the resonator, which is typicallya crystal. In an alternative embodiment, two ADC-s are between therespective phase detectors and loop filters. In even another embodiment,the two ADC-s are between respective loop filters and controlledfrequency dividers.

The first frequency phase detector 150 takes as inputs the first dividedsignal 115 and the digital second signal 145 and provides as output thefirst phase difference signal 155. The second frequency phase detector151 takes as inputs the second divided signal 116 and the digital secondsignal 145 and provides as output the second phase difference signal156. The first loop filter 160 takes as input the first phase differencesignal and provides as output the first control signal 107. The secondloop filter 161 takes as input the second phase difference signal andprovides as output the second control signal 108.

Typically, the first divided frequency and second divided frequency aredifferent frequencies and both resonance frequencies of the resonator.Thus, the resonator is typically a resonator allowing resonances ofdifferent frequencies at the same time. Typically, the resonator is acrystal resonator. Furthermore, typically, at least one of theresonances is an overtone resonance, preferably both resonances areovertone resonances.

As described before, if at least one of the first and second dividedfrequencies are selected at an overtone frequency, the temperaturebehaviour may be different. The temperature may vary in a temperaturerange of −40° C. to +125° C. Within this range the temperature gradientfor a certain overtone may vary.

Either the first control signal or the second control signal may beselected as frequency ratio signal. Typically, the first control signaland the second control signal are combined to compensate for temperatureeffects, specifically temperature influences on the resonator. Thecombination is then provided as frequency ratio signal as output. Thecombination may be used to select the control signal that is the leastchanging for a certain temperature. As shown in and explained for FIG.6, the first and second control signals may be divided to provide atemperature indication allowing to compensate for any temperatureeffects of the frequency generator, frequency ratio generator andspecifically the resonator. The compensation is typically predefined,such as predefined during manufacturing by providing setting to thefrequency ratio generator to deduce a compensation factor for the firstand second control signal and/or other signal preferably internal to thefrequency ratio generator to provide a compensated frequency ratiosignal.

FIG. 5 schematically shows a third embodiment of a frequency ratiogenerator 102. The frequency ratio generator comprises all features asdescribed for FIG. 4. The circuit may further comprise a thirdcontrolled frequency divider 112, a third phase detector 152 and a thirdloop filter 162.

The first controlled frequency 110 divider takes as inputs the firstinput signal 105 and the first control signal 108 and provides as outputthe first divided signal 115. The second controlled frequency dividertakes as inputs a third input signal 105 and a second control signal 108and provides as output a second divided signal 116. The third controlledfrequency divider takes as inputs a fourth input signal 106 and a thirdcontrol signal 109 and provides as output a third divided signal 117.The adder takes as inputs the first, second and third divided signalsand provides as output an added signal 121. The added signal is theaddition of the first, second and third divided signals.

The first frequency phase detector 150 takes as inputs the first dividedsignal 115 and the digital second signal 145 and provides as output thefirst phase difference signal 155. The second frequency phase detector151 takes as inputs the second divided signal 116 and the digital secondsignal 145 and provides as output the second phase difference signal156. The third frequency phase detector 152 takes as inputs the thirddivided signal 117 and the digital third signal 145 and provides asoutput the third phase difference signal 157. The first loop filter 160takes as input the first phase difference signal and provides as outputthe first control signal 107. The second loop filter 161 takes as inputthe second phase difference signal and provides as output the secondcontrol signal 108. The third loop filter 162 takes as input the thirdphase difference signal and provides as output the third control signal109.

Typically, the first divided frequency, second divided frequency andthird divided frequency are different frequencies and all resonancefrequencies of the resonator. Thus, the resonator is typically aresonator allowing resonances of different frequencies at the same time.Typically, the resonator is a crystal resonator. Furthermore, typically,at least two of the resonances are an overtone resonance, preferably allresonances are overtone resonances.

As described before, if at least one of the first and second dividedfrequencies is selected at an overtone frequency, the temperaturebehaviour may be different. The temperature may vary in a temperaturerange of −40° C. to +125° C. Within this range the temperature gradientfor a certain overtone may vary.

Either the first control signal, the second control signal or the thirdcontrol signal may be selected as frequency ratio signal. Typically, thefirst control signal, the second control signal and/or the third controlsignal are combined to compensate for temperature and/or hysteresiseffects, specifically temperature and hysteresis influences on theresonator. The combination is then provided as frequency ratio signal asoutput. The combination may be to select the control signal that is theleast changing for a certain temperature. As shown in and explained forFIG. 7, the first and second control signals may be divided to provide afirst indication and the first and third control signals may be dividedto provide a second indication. Both or at least one allows tocompensate for any temperature effects of the frequency generator,frequency ratio generator and specifically the resonator. Bothindications, when subtracted, allow to compensate for any hysteresiseffects of the frequency generator, the frequency ratio generator andspecifically the resonator. The compensation is typically predefined,such as predefined during manufacturing by providing setting to thefrequency ratio generator to deduce a compensation factor for the first,second and third control signal and/or other signal preferably internalto the frequency ratio generator to provide a compensated frequencyratio signal.

FIG. 6 schematically shows a sub-system 200 for compensating temperatureeffects. The sub-system comprises a frequency ratio generator 101, 102according to any of the embodiments of the invention comprising theresonator and providing the control signal 107 and the second controlsignal 108, such as in FIGS. 1 and 2. The sub-system further comprises acontrol signal divider 220 arranged for generating a divided controlsignal 225 based on division of the control signal by the second controlsignal. The divided control signal is the temperature indication 225.The temperature indication may be used to apply a temperaturecompensation to the frequency ratio signal in a separate unit to providea temperature compensated frequency ratio.

FIG. 7 schematically shows a sub-system 201 for compensating temperatureand hysteresis effects. The system comprises a frequency ratio generator211 according to any of the embodiments of the invention comprising theresonator and providing the control signal 107, the second controlsignal 108 and the third control signal 109. The sub-system furthercomprises a first control signal divider 220 arranged for generating adivided control signal 225 based on division of the control signal bythe second control signal. The first divided control signal is the firsttemperature indication 225. The system further comprises a secondcontrol signal divider 221 arranged for generating a second dividedcontrol signal 226 based on division of the control signal by the thirdcontrol signal. The second divided control signal is the secondtemperature indication 226.

The system further optionally comprises a subtractor 230 arranged forgenerating a subtracted signal 235 based on subtracting the seconddivided control signal from the first divided control signal. Typically,the first divided control signal and the second divided control signalhave distinct minimum temperature activities. The subtracted signal isthe temperature indication and/or hysteresis indication. Based on thissubtracted signal the first, second and/or third control signals may becorrected for temperature and hysteresis influences, especially from theresonator. Furthermore, the frequency ratio signal may be compensatedfor temperature and hysteresis in a separate unit to provide atemperature and hysteresis compensated frequency ratio.

The embodiment in FIG. 4 combined with the feature that the embodimentis arranged to resonate at two distinct resonance frequencies, selectedsuch that the resonance frequencies have distinct activity dips, thefirst and second control signals, indicative for the first and secondratio respectively, would be suitable for measuring changes intemperature of the resonator, preferably a crystal resonator. Distinctminimum temperature activities in the context of this application areminimums having their minimum frequency change at different or distincttemperatures. This minimum change may be relative to another thefrequency of another signal. This embodiment provides the advantage ofbeing able to measure temperature changes of the resonator over thewhole temperature range with a high degree of accuracy. Anotheradvantage is that dividing of the first ratio by the second ratioprovides a ratio independent of the first frequency. Thus, anytemperature dependent variation of the first frequency may beeliminated.

Furthermore, the embodiment in FIG. 4 may be extended to comprise athird loop as shown in FIG. 5. The third loop comprises a 3-thcontrolled frequency divider 112, a 3-th phase detector 152 and a 3-thloop filter 162 all arranged in a similar way as for the first andsecond loop. Further, this embodiment is extended with a first controlsignal divider generating a first divided control signal by dividing thecontrol signal by the second control signal. Further, this embodiment isextended with a second control signal divider generating a seconddivided control signal by dividing the control signal by the thirdcontrol signal. Further, this embodiment is extended with a subtractorarranged for generating a subtracted signal based on subtracting thesecond divided control signal from the first divided control signal,wherein the subtracted signal is indicative of the temperature of theresonator. Further, at least the first divided control signal and thesecond divided control signal have distinct minimum temperatureactivities.

This embodiment provides the advantage of being able to measuretemperature changes of the resonator over the whole temperature rangewith a high degree of accuracy. Another advantage is that the divisionsof the ratios provide divided ratios, which are independent of the firstfrequency. Thus, any temperature dependent variation of the firstfrequency may be eliminated or at least minimized. Furthermore, anybehaviour of the resonator changing the resonance frequencies of theresonator, such as time dependent behaviour e.g. hysteresis, may becompensated.

The used frequency measurement technique allows measurement accuracy inthe order of 0.1 ppb at a speed of 1 k samples/s. This shifts to thatthe system is advantageously capable of measuring, with a stable firstfrequency, of small temperature changes of the resonator, such ascrystal. Small temperature changes may be in the range of milli-Kelvin.Further, the system is typically capable to respond sufficiently fast tochanges.

In an embodiment of the frequency ratio generator, an offset is added tothe phase difference signal. This allows for letting the loop lock on adifferent angle. For example, if the frequency phase detector has aminimum output signal for 0-degrees phase shift on its inputs, theoffset will cause the loop to lock at non-0-degrees. For example, if thefrequency phase detector has a minimum output signal for 90-degreesphase shift on its inputs, the offset will cause the loop to lock atnon-90-degrees.

In an embodiment, a resonator, such as a crystal oscillator, crystal orcrystal resonator, may be used, which is indirectly read out. Thisindirectly reading out may introduce a phase shift. This introducedphase shift may be corrected for with the introduction of an offset asdescribed above.

One or more of the signals are typically normalized to simplify furthercalculations with these signals.

In a variation of the frequency generator, the frequency ratio signal isbased on the first phase difference signal, the second phase differencesignal and/or the third phase difference signal. The selection from orweight of each of the phase difference signals may still be based on thefirst control signal, the second control signal and/or the third controlsignal. This variation provides the advantage that the inner loopsignals are only filtered by the inner loop filter and the outer loopsignals are only filtered by the outer loop filter. Each of the filters,especially the outer loop, may be tailored to the specific requirementsof that loop. The requirements for each loop typically strike a balancebetween stability of the loop and agility to outside changes.

In a variation of the frequency generator, the outer loop filter 320 andthe adder 356 are swapped, such that the offset signal passes alsothrough the outer loop filter. This variant has the advantage ofadditionally filtering the offset signal to further stabilize thissignal. The original configuration, as shown in FIG. 2, provides theadvantage of solely filtering the signals through the PLL, such that theoptimal balance can be reached or approached between stability andagility of this loop formed by the controlled oscillator and the PLL andalso for the loop formed by the controlled oscillator, phase acquisitioncircuit and the PLL.

To facilitate the collaboration between the different blocks mentionedthroughout this description and shown in the figures, it might benecessary to scale, shift signals and/or inverted signals. For example,the frequency ratio signal and the target ratio are typically invertedin the comparator to come to a comparison signal, which is suitable tobe processed by the rest of the system.

FIG. 8 schematically shows an embodiment of a computer program product1000, computer readable medium 1010 and/or non-transitory computerreadable storage medium comprising a computer readable code 1020. Thecomputer readable code implements methods mentioned throughout thedescription according to the invention.

A signal may be periodic signals. A periodic signal repeats itself afterevery period. The amount of repetitions per second equals the frequency.Furthermore, a signal may have a maximum magnitude, such as an amplitudeor a value, an average signal level and an RMS level. A signal in thecontext of this text may be an analogue signal, such as a voltagesignal, a current signal, a power signal and/or an energy signal. Asignal in the context of this text may also be a digital signalrepresenting a voltage signal, a current signal, a power signal and/oran energy signal. The frequency ratio is a frequency ratio signal.

A controlled frequency divider may be a digital controlled frequencydivider. The controlled frequency divider may be a controlled multiplefrequency divider. The controlled multiple frequency divider provides anoutput signal having an output frequency equal to an input frequency ofan input signal divided by n, wherein n is a number of the collection N.In formula:

${f_{out} = \frac{f_{i\; n}}{x}};{x \in {\mathbb{N}}^{+}}$

Alternatively, the controlled frequency divider may be a controlledfractional divider. In formula:

${f_{out} = \frac{f_{i\; n}}{x}};{x \in {\mathbb{Q}}^{+}}$in practical implementations, the controlled fractional frequencydivider may be limited to for example:

${f_{out} = \frac{f_{i\; n}}{x}};{x \in {\mathbb{Z}}^{+}}$

An exemplary embodiment of a digital controlled multiple frequencydivider is, wherein the divider switches between a division number N andN+1. A prerequisite is that if the divided signal is fed to a resonator,this resonator has a decent quality Q. By switching between N and N+1,for instance fractional numbers such as N+¾ or N+ 5/7 are possible.

The implementation of such a digital controlled multiple frequencydivider may be done with the addition of an accumulator with a settablemaximum value. In the example of N+¾ an accumulator with a maximumcapacity of 4 and a repeatedly added number of 3 will have a carry in 3out of the 4 cycles. Each time the carry is present the divider shoulddivide by the N+1 number, any other time the divider should divide by N.This technique may be typed as shaping.

Higher order shaping may be done by adding another accumulator and asmall differentiator. As a result, for higher order shaping the digitalcontrolled frequency divider may divide by either N−1, N, N+1 or N+2.Higher order shaping causes the spectral behaviour to show a steeperroll off causing less noise in the loop. Higher order shaping providesthe circuit with the advantage of a more stable frequency ratio.

A DTC (Digital to Time Converter) may be used to shift the edge of asignal, such as the output of a digital controlled multiple frequencydivider. The digital controlled multiple frequency divider divides Nalternated in some pattern by division by N+1, while the DTCinterpolates the edges to be on near perfect time. A DTC therefore maydecrease the jitter introduced in the circuit providing the advantage ofa more stable circuit and/or frequency ratio.

The two methods above, being a digital controlled multiple frequencydivider and a DTC, have different performance in the sense of noise andaccuracy. The digital controlled multiple frequency divider provides adivided signal wherein the loop stability and optionally accuracy relyon filtering rejection by the loop filter and if present on theresonator. On the other hand, the DTC provides much better initialaccuracy, but has as a disadvantage that it adds a spectrally largenoise component. Which advantage or disadvantage is dominant is noteasily determined as the advantages and disadvantages vary with thebehaviour of the other circuit components, specifically with the othercircuit components in the loop, and the signals provided to the circuit.

The frequency phase detector may be a frequency mixer, analoguemultiplier, digital circuit or logic circuit configured as phasedetector. A frequency phase detector, phase detector or phase generatesan output signal, such as a phase difference signal, which representsthe difference in phase between two input signals, such as between adivided signal and a second input signal. Depending on the type offrequency phase detector, the input signals may need to be phase shiftedto provide an output signal which may be used to lock on for the loop.As an example, a logic circuit phase detector, made from ex-OR logicgates, typically locks the loop at a 90° degree phase shift between theinput signals.

The inner and outer loop filters stabilize the inner and outer loop,respectively. The inner and outer loop filters may further stabilize therespective loop in view of the input signals provided to the frequencyratio generator and frequency generator, respectively. If a resonator ispresent, the inner and outer loop filters may further stabilize thecircuit taking into account the resonator behaviour. The inner and outerloop filters may be a first or multiple order filter. The loop filtersare typically low-pass filters. The cut-off frequency of theloop-filters are typically a balance between accuracy and speed ofcorrection of disturbances in the circuit. A lower cut-off frequencyprovides more accuracy as less jitter is allowed trough the respectiveloop filter, while a higher cut-off frequency provides a quickerresponse to changes in the circuit, such as temperature changes. Also,the respective loop lock behaviour of the circuit may be influenced bythe respective loop filter, specifically the selection of the cut-offfrequency. An important factor when designing a loop filter is toconsider the loop gain. The respective loop filter is typicallyimplemented as a PID controller.

A resonator has a fundamental frequency, which is the lowest frequencyof resonance. Further, the resonator may resonate at harmonicfrequencies, which adhere to the relationf _(harmonic) =f _(fundamental)*2πx;x∈

⁺

Further, the resonator may resonate at an overtone frequency, whichadheres to the relationf _(overtone) =f _(fundamental)*(2πx+y);x∈

⁺,0<y<<2π

Resonance frequencies of a crystal, a crystal oscillator or crystalresonator may be even or odd harmonics and the associated overtones.Typically, the odd harmonics and associated overtones are used forletting crystals resonate.

It should be noted that the figures are purely diagrammatic and notdrawn to scale. In the figures, elements which correspond to elementsalready described may have the same reference numerals.

It will be appreciated that the invention also applies to computerprograms, particularly computer programs on or in a carrier, adapted toput the invention into practice. The program may be in the form of asource code, an object code, a code intermediate source and an objectcode such as in a partially compiled form, or in any other form suitablefor use in the implementation of the method according to the invention.It will also be appreciated that such a program may have many differentarchitectural designs. For example, a program code implementing thefunctionality of the method or system according to the invention may besub-divided into one or more sub-routines. Many different ways ofdistributing the functionality among these sub-routines will be apparentto the skilled person. The sub-routines may be stored together in oneexecutable file to form a self-contained program. Such an executablefile may comprise computer-executable instructions, for example,processor instructions and/or interpreter instructions (e.g. Javainterpreter instructions). Alternatively, one or more or all of thesub-routines may be stored in at least one external library file andlinked with a main program either statically or dynamically, e.g. atrun-time. The main program contains at least one call to at least one ofthe sub-routines. The sub-routines may also comprise function calls toeach other. An embodiment relating to a computer program productcomprises computer-executable instructions corresponding to eachprocessing stage of at least one of the methods set forth herein. Theseinstructions may be sub-divided into sub-routines and/or stored in oneor more files that may be linked statically or dynamically. Anotherembodiment relating to a computer program product comprisescomputer-executable instructions corresponding to each means of at leastone of the systems and/or products set forth herein. These instructionsmay be sub-divided into sub-routines and/or stored in one or more filesthat may be linked statically or dynamically.

The carrier of a computer program may be any entity or device capable ofcarrying the program. For example, the carrier may include a datastorage, such as a ROM, for example, a CD ROM or a semiconductor ROM, ora magnetic recording medium, for example, a hard disk. Furthermore, thecarrier may be a transmissible carrier such as an electric or opticalsignal, which may be conveyed via electric or optical cable or by radioor other means. When the program is embodied in such a signal, thecarrier may be constituted by such a cable or other device or means.Alternatively, the carrier may be an integrated circuit in which theprogram is embedded, the integrated circuit being adapted to perform, orused in the performance of, the relevant method.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or stages other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

Examples, embodiments or optional features, whether indicated asnon-limiting or not, are not to be understood as limiting the inventionas claimed.

The invention claimed is:
 1. A frequency generator for generating acontrolled signal having a controlled frequency, comprising: a frequencyratio generator arranged for generating a frequency ratio, comprising:an input configured for receiving the controlled signal; a firstcontrolled frequency divider arranged for generating a first dividedsignal having a first divided frequency being substantially thecontrolled frequency divided by a first frequency ratio signal; aconverter arranged for generating an excitation signal having the firstdivided frequency based on the first divided signal, wherein theexcitation signal is provided to the resonator for excitation of theresonator; a resonator arranged for generating a resonance signal havinga first resonance frequency, wherein the resonator is excited by theexcitation signal; a first frequency phase detector arranged forgenerating a first phase difference signal based on a first frequencyphase difference between the first divided frequency and the firstresonance frequency; a first inner loop filter arranged for generatingthe first frequency ratio signal; and an output configured for providinga frequency ratio signal based on the first frequency ratio signalindicative of the frequency ratio between the controlled frequency andthe first resonance frequency; wherein a first frequency ratio generatorloop is formed by the first controlled frequency divider, the firstdivided signal, the first frequency phase detector, the first phasedifference signal, the first inner loop filter and the first frequencyratio signal; wherein a second frequency ratio generator loop is formedby the first controlled frequency divider, the first divided signal, theconverter, the excitation signal, the resonator, the resonance signal,the first frequency phase detector, the first phase difference signal,the first inner loop filter and the first frequency ratio signal; andwherein the first inner loop filter filters the first phase differencesignal such that instability of the frequency ratio generator loops isprevented; and a comparator arranged for generating a comparison signalbased on the comparison of the frequency ratio with a target ratio; anda controlled oscillator circuit arranged for generating the controlledsignal having the controlled frequency based on the comparison signal.2. The frequency generator according to claim 1, further comprising: anouter loop filter arranged for generating a filtered comparison signalbased on the comparison signal; wherein the controlled frequency isbased on the filtered comparison signal; wherein an outer loop is formedby the frequency ratio generator, the comparator, the comparison signal,the outer loop filter, the filtered comparison signal, the controlledoscillator and the controlled signal; and wherein the outer loop filterfilters the comparison signal such that instability of the outer loop isprevented.
 3. The frequency generator according to claim 1, wherein thecomparator comprises a subtractor arranged for providing the comparisonsignal based on subtracting the target ratio from the frequency ratio;or wherein the comparator comprises a divider arranged for providing thecomparison signal based on dividing of the frequency ratio by the targetratio.
 4. The frequency generator according to claim 3, wherein thefrequency ratio generator comprises: a second controlled frequencydivider arranged for generating a second divided signal having a seconddivided frequency being substantially the controlled frequency dividedby a second frequency ratio signal; a second frequency phase detectorarranged for generating a second phase difference signal based on asecond frequency phase difference between the second divided frequencyand the second resonance frequency; a second inner loop filter arrangedfor generating the second frequency ratio signal indicative of thesecond frequency ratio based on the second phase difference signal; anadder arranged for generating an added signal supplied to the converter,wherein the added signal has the first divided frequency and the seconddivided frequency; a temperature compensator for compensating changes inresonance frequency of the resonator, wherein the temperaturecompensator comprises: an input configured for receiving the firstfrequency ratio and the second frequency ratio; a first divider arrangedfor generating a first division value based on dividing of the firstfrequency ratio by the second frequency ratio; a calculator arranged forcalculating a compensation factor based on the value of one or more ofthe group of the first phase difference signal, the second phasedifference signal, the first frequency ratio and the first divisionvalue, wherein the calculator comprises a look-up table; and a redresserarranged for generating the frequency ratio based on redressing a signalbased on one or more of the group of the first phase difference signal,the second phase difference signal and the first frequency ratio withthe compensation factor; wherein the excitation signal is based on theadded signal; wherein a third frequency ratio generator loop is formedby the second controlled frequency divider, the second divided signal,the second frequency phase detector, the second phase difference signal,the second inner loop filter and the second frequency ratio signal;wherein a fourth frequency ratio generator loop is formed by the secondcontrolled frequency divider, the second divided signal, the converter,the excitation signal, the resonator, the resonance signal, the secondfrequency phase detector, the second phase difference signal, the secondinner loop filter and the second frequency ratio signal; and wherein thesecond inner loop filter filters the second phase difference signal suchthat instability of the frequency ratio generator loops is prevented. 5.The frequency generator according to claim 4, wherein the frequencyratio generator further comprises: a selector arranged for selecting oneor more of the group of the first phase difference signal, the secondphase difference signal and the first frequency ratio based on the valueof one or more of the group of the first frequency ratio and the firstdivision value; wherein the redresser is arranged for generating thefrequency ratio based on redressing the selected frequency ratio withthe compensation factor.
 6. The frequency generator according to claim4, wherein the frequency ratio generator further comprises: a thirdcontrolled frequency divider arranged for generating a third dividedsignal having a third divided frequency being substantially thecontrolled frequency divided by a third frequency ratio signal; a thirdfrequency phase detector arranged for generating a third phasedifference signal based on a third frequency phase difference betweenthe third divided frequency and the third resonance frequency; a thirdinner loop filter arranged for generating the third frequency ratiosignal indicative of the third frequency ratio based on the third phasedifference signal; wherein the added signal also has the third dividedfrequency; wherein the input of the temperature compensator is furtherconfigured for receiving the third frequency ratio; and wherein thetemperature compensator further comprises: a second divider arranged forgenerating a second division value based on dividing of the firstfrequency ratio by the third frequency ratio; wherein the selector isfurther arranged for selecting also from the group of the third phasedifference signal, the second frequency ratio and the third frequencyratio and also based on the value of one or more of the extended groupof the second frequency ratio, the third frequency ratio and the seconddivision value; wherein the calculator is further arranged forcalculating the compensation factor based on the value of one or more ofthe group extended with the third phase difference signal, the secondfrequency ratio, the third frequency ratio and the second divisionvalue; wherein the redresser is arranged for generating the frequencyratio based on redressing the selected frequency ratio with thecompensation factor; wherein a fifth frequency ratio generator loop isformed by the third controlled frequency divider, the third dividedsignal, the third frequency phase detector, the third phase differencesignal, the third inner loop filter and the third frequency ratiosignal; wherein a sixth frequency ratio generator loop is formed by thethird controlled frequency divider, the third divided signal, theconverter, the excitation signal, the resonator, the resonance signal,the third frequency phase detector, the third phase difference signal,the third inner loop filter and the third frequency ratio signal; andwherein the third inner loop filter filters the third phase differencesignal such that instability of the frequency ratio generator loops isprevented.
 7. The frequency generator according to claim 6, wherein theselector of the frequency ratio generator is arranged for: selecting oneof the group of the first frequency ratio, the second frequency ratioand the third frequency ratio, wherein the selected ratio is based onthe value of one or more of the group of the first frequency ratio, thesecond frequency ratio, the third frequency ratio, the first divisionvalue and the second division value; or selecting a weighted combinationof two or more of the first frequency ratio, the second frequency ratioand the third frequency ratio, wherein the weighted combination is basedon the value of one or more of the group of the first frequency ratio,the second frequency ratio, the third frequency ratio, the firstdivision value and the second division value.
 8. The frequency generatoraccording to any of claim 4, wherein the first division value is solelybased on the first frequency ratio signal and the second frequency ratiosignal; wherein the second division value is solely based on the firstfrequency ratio signal and the third frequency ratio signal; wherein theselector is arranged for selecting one or more of the limited group ofthe first phase difference signal, the second phase difference signaland the third phase difference signal; or wherein the calculator isarranged for calculating a compensation factor based on the value of oneor more of the limited group of the first division value and the seconddivision value.
 9. The frequency generator according to claim 8, whereinthe frequency ratio generator further comprises an analog-to-digitalconverter arranged for generating a digital resonance signal based onthe resonance signal, wherein the digital resonance signal is suppliedto at least the first frequency phase detector.
 10. The frequencygenerator according to claim 9, wherein the converter of the frequencyratio generator comprises a digital-to-analog converter arranged forgenerating the excitation signal based on the divided signal.
 11. Thefrequency generator according to claim 1, wherein the comparatorcomprises: a scaler arranged for generating a scaled signal, which isthe frequency ratio signal scaled by a scaling factor; or shifterarranged for generating a shifted signal, which is the scaled signalshifted by a shift value; wherein the comparison signal is based on theshifted signal.
 12. The frequency generator according to claim 1,comprising: a phase acquisition circuit arranged for generating a phasedelta signal based on the phase difference between the controlledfrequency and a reference signal having a reference frequency; and a PLL(Phase Locked Loop) arranged for generating an offset signal based onthe phase delta signal; wherein the controlled frequency is also basedon the offset signal.
 13. The frequency generator according to claim 12,wherein the comparator comprises a shifter arranged for generating ashifted signal, which is the scaled signal shifted by a shift value, andthe shift value is the offset signal; or further comprising an outerloop filter arranged for generating a filtered comparison signal basedon the comparison signal, wherein the filtered comparison signal isindirectly based on the offset signal.
 14. A frequency generator systemcomprising the frequency generator according to claim 1, and furthercomprising: a frequency generator chip including the frequencygenerator; a resonator for connecting to the frequency generator chipfor generating the resonance signal; and an oscillator for connecting tothe controlled oscillator circuit for generating an oscillating signal.15. A method for generating a controlled signal having a controlledfrequency, comprising the steps of: receiving a resonance signal havinga first resonance frequency from a resonator; providing a first ratiosignal indicative of the first frequency ratio between the controlledfrequency and the first resonance frequency; generating a first phasedifference signal based on a first frequency phase difference between afirst divided frequency and a first resonance frequency; filtering thefirst phase difference signal for generating the first frequency ratiosignal; such that instability of the frequency ratio generator loops areprevented; basing a frequency ratio on the first frequency ratio signal;providing a target ratio; generating a comparison signal based on thecomparison of the frequency ratio with the target ratio; generating thecontrolled signal having the controlled frequency based on thecomparison signal; generating a first divided signal having a firstdivided frequency being substantially the controlled frequency dividedby the first frequency ratio signal; generating an excitation signalhaving the first divided frequency based on the first divided signal,wherein the excitation signal is provided to the resonator forexcitation of the resonator; and outputting the controlled signal;wherein a first frequency ratio generator loop is formed by the firstdivided signal, the first phase difference signal and the firstfrequency ratio signal; wherein a second frequency ratio generator loopis formed by the first divided signal, the excitation signal, theresonator, the resonance signal, the first phase difference signal andthe first frequency ratio signal; and wherein the step of filteringfilters the first phase difference signal such that instability of thefrequency ratio generator loops are prevented.
 16. The method of claim15, wherein Computer a computer program product comprising a computerreadable medium having computer readable code embodied therein isconfigured such that, on execution by a suitable computer or processor,the computer or processor is caused to perform the steps of providingthe first ratio signal indicative of the first frequency ratio betweenthe controlled frequency and the first resonance frequency; basing thefrequency ratio on the first frequency ratio signal; generating thecomparison signal based on the comparison of the frequency ratio withthe target ratio; generating the controlled signal having the controlledfrequency based on the comparison signal; generating the excitationsignal having the first divided frequency based on the first dividedsignal, wherein the excitation signal is provided to the resonator forexcitation of the resonator; and outputting the controlled signal.
 17. Acomputer program product comprising a computer readable medium havingcomputer readable code embodied therein, the computer readable codebeing configured such that, on execution by a suitable computer orprocessor, the computer or processor is caused to perform the method ofclaim 15.